Evidence adds up: three studies of human impact on climate

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John Cook does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

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Three new studies were published today, each looking at a different aspect of the human impact on climate, each carrying a sobering message on the consequences of human activities on our environment. The first says recent warming is unprecedented in 2,000 years. A second reports climate zones are shifting faster due to warming temperatures. The third argues impacts from greenhouse gas emissions are not caused solely by warming temperatures.

The one that will likely attract the most attention is a reconstruction of global temperature over the last 2,000 years. This was a mammoth collaboration, involving 78 scientists from 60 scientific institutions, all part of the PAGES network (Past Global Changes). This network enabled researchers to use ice cores, tree-rings, lake sediments and other forms of data from all over the globe. The result - the PAGES 2k Paper - is a robust reconstruction of temperature across seven continents over 2,000 years.

They found that over the last 2,000 years, the planet had been gradually cooling. This cooling trend reversed around the time that humans started emitting heat trapping gases into the atmosphere. Since then, global temperatures have been rising, with the last few decades the warmest in 1,400 years.

The PAGES 2K paper also found that while the planet as a whole is experiencing unprecedented temperatures in recent decades, some pre-industrial regions were warmer than now. For example, Europe was possibly warmer during the Roman Warm Period. However, different regions warmed at different times. The modern period is the only time that all regions warmed simultaneously.

Another paper published today in Nature Climate Change examined the shifting of climate zones due to warming temperatures. They found that due to warming, climate zones are moving at an increasing pace. If humans continue to emit greenhouse gases at the current rate, the speed with which climate zones are shifting will double by the end of the century. This means about 20% of all land area will undergo a change.

Shifting climate zones could leave us high and dry.Flickr/thoughtfactory

One suggested magic bullet to minimise the impacts of human-caused global warming is geoengineering. For example, putting tiny sulphate particles into the atmosphere could reflect incoming sunlight. However, a third study published today, in Nature Geoscience, found some impacts from greenhouse gas emissions cannot be fully mitigated by geoengineering. A simulation of business-as-usual greenhouse gas emissions over the 21st century found that much of the changes in regional rainfall weren’t related to surface warming. Instead, the extra carbon dioxide in the atmosphere was causing shifts in atmospheric vertical motions, which led to changes in tropical rainfall.

This means that even if geoengineering managed to cancel out surface warming, rainfall patterns would still change because of the extra carbon dioxide building up in the atmosphere. There are other consequences of the increased carbon dioxide, beyond surface warming. For example, the oceans are absorbing around half of our carbon emissions, which is having a negative impact on coral reefs due to ocean acidification.

Research into the impacts of human-caused global warming continues to mount. A Web of Science search for papers matching the terms “global climate change” or “global warming” finds an accelerating amount of climate research.

Number of papers per year matching the Web of Science search ‘global climate change’ or ‘global warming’.

The consensus gap: the divergence between public perception of scientific consensus and the 97% reality.

Despite the steady accumulation of studies documenting the human impact on climate change, misinformation disseminated by a small number of climate misinformers has had an impact on how the public think about climate change. In my own research, I measured the public perception of scientific consensus by asking a representative US sample how many scientists agree that humans are causing global warming. The average answer was a shade under 50%. This is in stark contrast to two recentstudies that independently found a 97% consensus.

As more research like today’s slew of new papers are published, we can expect climate misinformers to continue to deny the full body of evidence. Inconvenient studies will be attacked and data will be cherry picked. The public need to understand that on a fundamental topic such as human-caused global warming, the attacks don’t come from genuine scientific debate but an attempt to generate false controversy.

The scientific consensus on human-caused global warming was established 20 years ago. It has only strengthened as the evidence continues to accumulate.